The present invention relates to a method and apparatus for decoding one-dimensional (1D) bar code symbols, and is directed more particularly to a method and apparatus for decoding 1D bar code symbols that have been imaged by 1D imaging-type bar code readers which have optical assemblies that include cubic phase masks.
1D bar codes, also known as linear bar codes, have become so widely used in point-of-sale, inventory control, shipment tracking, and other applications that many types of optical readers and optical reading systems have been developed to accommodate them. Generally speaking, these optical readers and reading systems may be divided into two broad types. A first of these types, known as laser scanning readers, makes use of a laser beam which is swept across the symbol to be read. In such readers, the intensity of the light reflected back from the target is measured by a photodetector. The output of the photodetector is then digitized, often in real time, to produce a digital signal for use in decoding the bar code symbol. A second of these types, referred to herein as imaging-type bar code readers, make use of an unswept, elongated illuminating beam. In readers of this type, an image of the target is formed on a 1D image sensor, which may be of any of a variety of types, including CCD and CMD image sensors, among others. When the imaging process is complete, the output signal of the image sensor is binarized to produce a two state signal for use in decoding the bar code symbol.
An important consideration in the design of both types of 1D optical readers is their ability to resolve the bars and spaces of the target in spite of changes in the distance between the reader and its target, i.e., to provide a good resolution over a substantial depth of field. In practice, laser scanning bar code readers usually provide a greater resolution and a greater depth of field than imaging-type bar code readers. This is because laser beams can be formed into narrow beams, and because their photodetectors need not include image forming optical elements. While the depth of field that can be achieved with imaging-type bar code readers can be increased by reducing the size of their apertures, such reductions give rise to other problems, such as reduced image intensity and an increase in aperture related diffraction effects. The depth of field that can be achieved with laser scanning bar code readers is also limited, however, since laser beams tend to spread as they propagate, and thereby progressively lose their ability to resolve closely spaced bars and spaces.
Prior to the present invention, a number of attempts have been made to improve the resolution and depth of field of both laser scanning and imaging bar code readers, some of which have involved the use of phase masks. In the case of laser scanning readers, phase masks have been used to shape the laser beam so as to hold the width thereof relatively constant over the desired depth of field. An example of a laser scanning reader which includes such a phase mask is shown and described in U.S. Pat. No. 5,646,391 (Forbes et al).
In the case of imaging-type bar code readers, the use of phase masks to improve depth of field is more complex. In such readers phase masks are included in the imaging optical assemblies to make the Optical Transfer Functions (OTFs) of those optical assemblies relatively invariant over the required depth of field. Because the images formed on the image sensors of these optical assemblies (often referred herein to as intermediate images) are the result of the superposition of the point spread functions (PSFs) of the points of the objects being imaged, they are too distorted to be used for their intended purpose without first being converted to final images that were corrected for the effect of the phase mask. One way of making this correction was to deconvolve the incoherent OTF of the phase mask from the intermediate image signal produced by the image sensor. An apparatus of this general type which is suitable for use in general purpose imaging systems is shown and described in U.S. Pat. No. 5,748,371 (Cathey, Jr. et al).
An apparatus of the last mentioned general type which is specially adapted for use in imaging-type bar code readers is shown and described in copending U.S. patent application Ser. No. 09/113,523, filed Jul. 10, 1998 (Hammond), which is commonly assigned herewith. In the latter application, an improved, generalized recovery function is used to correct a frequency domain representation of the intermediate image signal, and the result transformed back into the spatial domain to produce a final image signal. This final image signal is then digitized in order to place it in a form in which it may be decoded. While readers of this type produce excellent results, particularly when used with optical assemblies that are optimized for bar code reading applications, they have the disadvantage that they must be able to apply the Discrete Fast Fourier Transform (DFFT) and Inverse Discrete Fast Fourier Transform (IDFFT), and to multiply and divide large numbers of complex numbers. Providing a reader with the ability to perform these operations not only greatly increases the complexity of the reader software, it also slows down the reading process and thereby decreases the reader's overall data throughput rate.
In view of the foregoing, it will be seen that there exists a need for an imaging-type optical reader which has a large depth of field, but which does not achieve this depth of field by using complex mathematical operations that substantially reduce the reader's overall data throughput rate.